WO 2006/091181 A1 discloses an optical system.
By way of example the optical system is used for optical imaging, in particular field imaging and/or pupil imaging, which play a substantial role in microscopy and microlithography. The optical properties of the microscopic or microlithographic apparatuses depend primarily on the quality of the optical imaging with the optical system present there.
A typical optical system for optical imaging regularly has at least one optical element, which consists of at least one light-transmissive material. This material has a refractive index that depends on the wavelength of the light incident on the optical element. This wavelength-dependence of the refractive index, which is also referred to as dispersion, leads to chromatic aberrations in refractive optical elements, for example in an optical lens element which has a characteristic focal position in respect of a certain wavelength. Chromatic aberrations are understood to be optical aberrations that can be traced back to the wavelength-dependent refractive power of the lens element. The wavelength-dependence of the focal position of an optical lens element arises on account of light with different wavelengths or colors being refracted to a different extent by the optical lens element. In photography, green and red color fringes arise in recordings, in particular at light/dark transitions, the color fringes being referred to as transverse chromatic aberrations, with the image additionally appearing out of focus, which is known as a longitudinal chromatic aberration.
These chromatic aberrations can be subdivided further into so-called primary and secondary aberrations. If an aberration relates only to the differences in the imaging in the case of two different wavelengths, it is a primary aberration, while aberrations relating to more than two wavelengths are secondary aberrations.
In order to counteract chromatic aberrations, use is made of an achromat in order to obtain an identical focal position for different wavelengths. However, using a simple achromat made out of two different materials, it is generally only possible to obtain an identical focal position for two wavelengths. Here, the primary longitudinal chromatic aberration and the primary transverse chromatic aberration can be corrected by this achromat. However, the focus of a wavelength lying therebetween deviates to a greater or lesser extent from this focal position depending on the dispersion properties of the employed materials. In the case of very high demands in microscopy or microlithography, it is desirable, in particular, to also correct the secondary longitudinal chromatic aberration (also referred to as the secondary spectrum).
These days, the secondary spectrum can be minimized by apochromatic lens elements, with a skillful selection of the lens element materials, in particular of those with anomalous partial dispersion, having to be made. Partial dispersion is understood to be the ratio of the differences between the refractive index of one lens element material in respect of two different wavelength pairs, with different lens element materials having different partial dispersion characteristics. However, this option does not exist in the UV range, in which only very few optical materials are available. Therefore, U.S. Pat. No. 5,754,340 proposes to reduce the secondary spectrum by a combination of lens elements made out of quartz glass and/or calcium fluoride with a diffractive optical element, constituting a very complicated solution.
The literature has disclosed that the secondary spectrum can also be corrected if use is made of only two lens element materials, provided the design parameters of the system are selected appropriately. The publication C. G. Wynne, “A comprehensive first-order theory of chromatic aberration. Secondary spectrum correction without special glasses”, Optica Acta: International Journal of Optics, 25 (1978), pages 627-636 constitutes an example hereof.
The document cited at the outset discloses a lens element system having three lens element groups which only consist of two lens element materials. The lens element system facilitates a sufficiently good correction of various aberrations, in particular of the secondary longitudinal chromatic aberration in the visible spectral range. However, only imaging from the object (here at infinity) into the image plane is corrected well in this case, as is conventional and sufficient in many applications, whereas the pupil imaging is uncorrected from a color point of view. By way of example, for the system shown in FIG. 1 of the aforementioned document with a focal length of approximately 560 mm and an aperture of 80 mm, the image-side angles of incidence for wavelengths of 436 nm and 656 nm differ by approximately 10% in the case of a field angle of 1° and an image height of 9.5 mm.
DE 101 13 612 A1 discloses a partial lens for illuminating an image field, in particular in an illumination device for a microlithographic projection exposure apparatus, wherein the partial lens consists of two lens element groups having one or two lens element materials. As a result, aberrations in field imaging and in pupil imaging are corrected. However, that secondary aberrations of the two imagings can likewise be corrected by the partial lens disclosed therein cannot be gathered from DE 101 13 612 A1.